It will be apparent that in numerous industrial and other processes it is essential to provide an instrument which is capable of producing an electrical signal indicative of the amount of solid material present in a sample of fluid. It has been known in the prior art for many years, for example, to provide such a signal indicative of the condition of lubricating oil in aircraft engines. See U.S. Pat. Nos. 3,736,431 to Childs; 3,627,423 to Knapp; or 3,892,485 to Merritt et al. Similarly, such apparatus is useful in water purification apparatus for determining whether all solid materials are being filtered from the water. See commonly assigned U.S. Pat. No. 4,145,279 to Selby which describes a water purification system in which the nephelometer of the present invention may play an important part.
Usually nephelometric measurements have been made by passing an incident beam of light of known intensity upon the sample to be tested and measuring either the amount of light transmitted through the sample or the amount scattered by the solid particles within the sample. See, e.g., U.S. Pat. No. 3,869,209 to Sigrist, or 3,936,192 to Skala. A distinction is sometimes drawn between the use of the terms "nephelometry", to indicate measurements made of the scattered light intensity, and "turbidimetry" to indicate the measurements of the intensity of the transmitted light. In this application, the term "nephelometry" shall be construed to include turbidimetry, unless the context clearly indicates otherwise, as certain of the improvements made by the present invention are applicable to both sorts of systems.
Nephelometric measurements have, in general, required several significant problems to be solved. Clearly, in order to measure the light either transmitted or scattered from a fluid sample, the sample tube must be transparent; if the tube's transparency varies over time due to, for example, the collection of dirt on either the inside or the outside of the tube, the measurement will vary over time, so that the instrument will require periodic cleaning and/or adjustment of its output to match a sample of known turbidity. This problem has been discussed in U.S. Pat. No. 3,861,198 but no adequate solution is suggested therein. Another problem which occurs is leakage into the light sampling tube of stray light from the surrounding environment. A further problem of numerous prior art systems is that they are very expensive to make due to the elaborate circuitry and mechanical construction required. A further problem with certain prior art nephelometers and turbidimeters is that the electric bulbs used to supply the incident light vary over time and moreover, do not have sufficiently long lifetimes to allow adequately trouble-free operation although measures have been taken to limit this problem; see U.S. Pat. No. 3,561,875 to Reid. Another problem which has occurred in numerous prior art nephelometric systems is that the instrument is incapable of distinguishing bubbles which are usually harmless in the fluid to be sampled from solid matter in the sample, thus giving erroneous indications of excess turbidity. A further problem inherent in certain prior art designs is that the photocells used to sample the turbidity of incident light only measure the intensity of the light falling on a small fraction of the sample and thus do not always provide accurate results.